A key requirement for the correct function of high precision machines is the temperature control of elements of the machine, and the temperature control of any fluids supplied to the machine. This is particularly true of, for example, roll turning machines and large optical grinding machines, where the dimensional stability of the machine structure must be maintained to fractions of a micrometer over extended periods of time.
Elements of the machine structure that typically must be temperature controlled include the machine base, linear guideways, workpiece spindles, tool spindles, metrology structures, and other parts which would influence the control of final workpiece dimensions. Different types of machine element present different challenges in the way their temperature, and thus dimensional stability, are controlled.
A machine base and the larger components of a machine are passive, and being generally the bulk of the machine structure must simply be held at a pre-determined temperature, typically 20° C. The acceptable limit to variations from that temperature can be as small as ±0.001° C. The entire machine will typically be placed in a temperature controlled environment. However this environment cannot be maintained to such a fine level of thermal stability without extraordinary effort and an accompanying uneconomic cost overhead.
A method that is known to give good results for maintaining the necessary stable structural control is executed by passing a temperature controlled fluid either over, or through the structure, where the fluid temperature is at the required temperature for the structure in question. This method has been referred to as a “Liquid Shower” or “Oil Shower” (where oil is the fluid). The liquid is caused to flow over the outer and sometimes inner surfaces of the structure. Other areas of the machine may have voids, or drillings, through which the temperature controlled fluid is also passed. This technology is described for example in a Society of Manufacturing Engineers paper entitled “An Order of Magnitude Improvement in Thermal Stability with Use of Liquid Shower on a General Purpose Measuring Machine”; J B Bryan et al, May 6, 1982.
As the temperature of the structure is overwhelmingly dominated by the temperature of the fluid, the fluid temperature must be maintained to the required temperature to the same level of accuracy as that of the machine structure, for example±0.001° C. in the aforementioned ultra-precision applications.
In other areas of a precision machine, particularly in fast moving elements such as wheel spindles, heat is generated by various means, due to inefficiencies of drive motors or from friction between two sliding surfaces for example. To remove the heat from these regions, and to prevent the heat entering the dimensionally critical regions of the machine (for example the base), a temperature controlled fluid is circulated through features such as drillings or cavities in the machine element in question, as close as is possible to the point of heat generation. Temperature control of this fluid with a high degree of accuracy is again desirable.
A further example of a fluid-related thermally critical machine function is where there is a requirement for the machine to be supplied with fluids related to the machine operating process, for example cutting fluids. These fluids are also required to be temperature controlled to high levels of accuracy as, again, they often come into contact with the machine elements previously described which define the machine's dimensional stability, for example the base. As these fluids are often switched on and off at various times to suit the machine operation, when the machine's accuracy requirement may be at its highest, the effect of varying fluid temperatures can be highly significant, again potentially requiring levels of control of ±0.001° C.
Another example of a fluid requiring a high level of temperature control relates specifically to machines which use hydrostatic bearings. In this case, the fluid that creates the stiff bearing films within spindles and linear slide systems often flows out of the bearings and over dimensionally critical machine elements, which would again be detrimental to the machine dimensional stability if the temperature of the fluid varied from that of the machine structure. In this particular case, the temperature of the fluid rises as it passes through the bearing, proportional to the pressure drop of the fluid, and as this pressure drop is known, and constant, the inlet temperature of the fluid can be lowered proportionally to exit at the machine ambient temperature.
Various methods have been employed to cause the fluid temperature to be changed. In some cases the machine working fluid is passed through a mechanism that directly heats or cools the fluid. In other cases a second controlling fluid at a higher or lower temperature to the machine fluid is passed through one side of a heat exchanger while the machine fluid passes through the other half, and by varying the flow rate of the controlling fluid the working fluid temperature is adjusted. In another method, a supply of machine fluid at a temperature higher than the required temperature is mixed with a supply of machine fluid at a lower temperature, the proportions being determined to achieve the required mixed temperature. This type of approach is described for example in US 2002/0020179 A1, WO 2008/078525 A1, and U.S. Pat. No. 1,873,769.
At higher accuracy levels, the required machine fluid temperature control is often not achieved by one system, but goes through a coarse temperature control device, followed by a single, or sometimes multi-step adjustment process, the final adjustment being made when the temperature variation is close to the required accuracy. In this way, the requirements for a system to have high heating or cooling power while simultaneously maintaining the highest levels of accuracy is avoided.